I would say you may have a bad thermostat, they have been known to sell bad ones from time to time. You can check it by placing it in a cup of water that is at the temp of the rating of the thermostat.
If it is a 195 degree thermostat, place it in a glass container of 195 degree water and see if it opens, then cool it and see if it closes.
If the thermostat is good you could either have vapor lock in the cooiling system, a blocked heater core or possibly a lazy water pump.